Continuous tone laser printers are used in medical diagnostic imaging applications to produce digital images on media, such as film, from image signals representing medical images. The laser printers are used with apparatus that provide image signals, such as computerized axial tomography, magnetic resonance, ultrasound and digital subtraction angiography.
The printers typically print picture elements, or pixels, on media of various sizes such as 8.times.10, 11.times.14, 14.times.14, and 14.times.17 inch, with a fixed pixel pitch, where pixel pitch is defined as the distance from midpoint of one pixel to the midpoint of the next adjacent pixel. The fixed pixel pitch is generally chosen such that 4096.times.5120 image pixels are printed or exposed on 14.times.17 inch media, with fewer pixels printed on smaller media sizes using the same pixel pitch. An example of the number of image pixels that can be printed on media of various sizes with the typical fixed pixel pitch of 80 microns, follows:
______________________________________ Media Size Image Pixels ______________________________________ 8 .times. 10 inch 2300 .times. 2900 11 .times. 14 inch 3232 .times. 4696 14 .times. 14 inch 4096 .times. 4096 14 .times. 17 inch 4096 .times. 5120 ______________________________________
Standard printer memory and image processing capabilities are designed to support a maximum of 4096.times.5120 pixels. Standard laser spot size is designed to compliment pixel pitch and maximize image quality and is generally twice the pitch of the pixels in the page or y-direction, and equal to the pixel pitch in the line or x-direction.
Typically, image signals provided from various medical apparatus will have an insufficient number of image pixels to fill the complete page on the desired size of media. In this situation interpolation is required to produce an image with a larger number of pixels. A variety of interpolation techniques have been used to achieve this. These interpolation techniques are well known to those skilled in the art and include replication, cubic convolution and bilinear interpolation. In replication a previous pixel value is repeated, forming a new pixel. In cubic convolution, a weighted sum is calculated from the two previous and two following rows as well as the two pixels to the right and left of the pixel being added (a 4.times.4 array). In bilinear interpolation the value of the created pixel is a weighted average of the four nearest pixels.